43rd Lunar and Planetary Science Conference (2012) 1410.pdf

GEOLOGIC AND GEOMORPHOLOGIC MAP OF IANI CHAOS (MARS). L. Guallini 1, M. S. Gilmore 2, L. Mari- nangeli 1, 1IRSPS, Università d’Annunzio, Pescara, Italy, [email protected] ; 2Wesleyan University, Middletown, CT, USA.

Introduction: Iani Chaos is a ~30,000 km 2 region including both Noachian bedrock and LLD deposits. that lies at the mouth of and is presumably the source for Five units have been distinguished with varying severity the Hesperian age Ares Vallis outflow channel system of modification (primarily by erosion and fracturing) [e.g., 1, 2]. Mapping of Ares Vallis reveals multiple starting from a common terrain (Noachian highlands). 1) episodes of erosion [3, 4]. Multiple discharge and re- Hummocky Terrain (Hy) : unit characterized by an ir- charge events have been proposed in the Iani Chaos regular hummocky topography (Fig. 1i). At CTX scale, aquifer system based on geophysical grounds [e.g. 5, 6], the unit displays a high-density pattern of smooth and causing collapses of the region. Detailed geologic and small knobs, mounds and mesas at the tens of meter to topographic characterization of the Iani Chaos has been kilometer, spaced out by minor order and shallow frac- developed by [12, 13, 14]. In particular, Light-toned turing systems, smoothed by the erosion; 2) Mesas Layered Deposits (LLD) have been partially mapped (Ms) : broad and flat plateaus bordered by wide and and described within it [7, 8], and are found to be com- deep extensional fracturing and faulting systems (gra- prised of sulfates [9, 10, 11] consistent with their for- bens; Fig. 1l); 3) Mesas & Knobs (MK) : broad plateaus mation by evaporation of water. bordered by wide and deep extensional fracturing and In the present work is presented the first high- faulting systems spaced out and/or overlapped by resolution geologic and geomorphologic map of the mounds (Fig. 1m); 4) Knobs (Kb) : groups of dome- region (Fig. 1a-o). shaped hills spaced out by marked valleys (tectonic Methodology: The region has been analyzed using and/or erosional) and continuous to the Ms and MK high-resolution visible images acquired by cameras on terrains; 5) Knobby Terrain (Ky) : small and diffuse the ESA Mars Express (HRSC orthoimages, 12.5 and mounds showing a smooth morphology and defining a 25.0 m/pix) and on the NASA MRO (CTX, 6.0 m/pix regional rough topography (Fig. 1n). and HiRISE, 0.25 m/pix). Topographic basemaps are Fluvial Channels Remnants (ChR) . Potential fluvial from MGS MOLA 128 pix/degree (460 m grid spacing) morphologies (channels, streamlined islands, terraces, and from HRSC DEMs Da4 (75 m and 125 m grid spac- grooved surfaces; cfr. Ch1 and Ch2 in Fig. 1o) are lo- ing). Imageries have been processed using the USGS cally exposed on the surface of the LLDs. Fluvial fea- Integrated Software for Imagers and Spectrometers tures are of similar orientation and elevation to channels (ISIS 3) and georeferenced into ArcGIS 9.x software at the mouth of the Ares Vallis; we interepret them to environment. CTX and HiRISE images are overlaid have once been a semi-continuous floodplain, later in- onto HRSC DTMs in ArcScene. terrupted by local chaos formation. Geologic Units (LLD) . At visible-light wave- Outflow Channels (ChS). Five main erosional sur- lenghts, LLD terrains (Fig. 1d) are clearly distinguished faces have been defined in Ares Vallis at the mouth of by a marked light-toned albedo and their morphology. Iani Chaos on the basis of their altitude and relative At CTX and HiRISE resolution they usually show a stratigraphic relationships (Fig. 1b, S5-S1 from young- polygonal texture down to the limit of resolution (Fig. est to oldest), and are in agreement with previous stud- 1e) and a meter scale “stair-stepped” layering (Fig. 1d). ies [3, 4]. The channel floors appear regionally flat but Karst-like landforms (i.e. doline-like fields) are also include several outflow erosional morphologies as exposed on their surface (Fig. 1f, g). The main dome- streamlined islands, grooved terrains, terraces and cata- shaped bodies fill the basins made by the collapsed racts. Their shape and topography indicates dominant chaotic terrains (Fig. 1o), occupying the valleys be- ~S to ~N flow direction. tween them. LLDs also overlap the mounds or are them- Topography: A detailed statistical topographic and selves eroded into mounds after deposition (point A in morphometric analysis has been developed using HRSC Fig. 1o). The LLDs are mostly modified by eolian proc- DEM elevations. esses (to form yardangs; Fig. 1h) and craters are rare or CsT Units. We observe a general progressive de- absent on their surface, indicating that these materials crease of mean elevation from the Hy , MS and MK ter- are eroded rapidly. rains (average of about -2200 m) to the Kb and Ky mor- Geomorphologic Units : Regional geomorphologic phologies (average of about -2900 m). This trend is units has been mapped at HRSC and CTX resolution consistent with a greater initial collapse of the original and are defined by depositional textures and/or ero- surface and an increase of the fracturing and/or of the sional patterns and albedo. erosional degree of terrains . Chaotic Terrains (CsT). These units are character- LLD and ChR Units. LLD N are characterized by ized by kilometer scale irregular block morphologies the lowest elevation (-3300 - -3500 m in average) of the 43rd Lunar and Planetary Science Conference (2012) 1410.pdf

mapped LLD deposits within Iani Chaos, being entirely younger and darker layers of LLD, entailing multiple within a basin. At the same time, it is also inferred to be episodes of deposition) and their partial tectonic tilting. characterized by a relatively thinner sequence and a Discussion and Conclusions: We have, for the first volume of ~1300 km 3. The LLD C have an higher time, mapped and described in detail geologic and weighted mean elevation (~-2700 m), almost covering geomorphologic units composing Iani Chaos. In particu- all the values comprised between the basins and the lar, we have defined LLDs physiography and chronol- chaotic interbasin highlands. Their maximum inferable ogy and we have identified likely fluvial features within volume is of ~6000 km 3. In terms of volume, the LLD S Iani Chaos, consistent with the topographic elevations are the third of the Iani Chaos basin (~900 km 3). Their of Ares Vallis outflow channels. These systems eroded weighted mean elevation is of -2500 - -2700 m, almost LLDs and controlled their elevation. We map at least close to the central LLD region. ChR elevations are two major episodes of LLD deposition that we suggest consistent with those of respective LLD region. Also, occurred in lakes that were the surface manifestation of LLDs are completely enclosed by higher topographic periodic recharging aquifer within Iani. relief, formed by the chaotic terrains. References: [1] Carr M. H. and Clow G. D. (1981) Stratigraphy . A relative stratigraphic relationship Icarus 48 , 91-117. [2] Grant J. A. and Parker T. J. (2002) between ChS in Ares Vallis, the LLD and the ChR has JGR 107 , 5066, doi: 10.1029/2001JE001678. [3] Pacifici, A. been derived from HRSC DEMs [13, 14]. From it, we et al. (2009) Icarus 202 , 60-77. [4] Warner N. et al. (2009) suggest that ChS outflows events controlled the LLD EPSL 288 , 58-69. [5] Carr M. H. (1979) JGR 84 , 2995- and ChR elevations and, thus, LLD was contemporane- 3007. [6] Andrews-Hanna J. C. and Phillips R. J. (2007) ous to Iani and Ares Vallis formation. In addiction, the JGR 112 , E08001. [7] T. Harrison (2008) M.S. Thesis , LLDs regionally overlap the CsT unit with a non- Wesleyan University. [8] Sefton-Nash E. (2010) LPSC 41 st , conformity contact. We interpret this stratigraphy to Abstract #1957. [9] Glotch T. D. and Rogers A. D. (2007) show that some LLD as remnants of an ancient body JGR 112 , E06001. [10] Noe Dobrea E. Z. et al. (2008) partially filling the tectonic valleys between the inter- Icarus 193 , 516-534. [11] Gilmore M. S. et al. (2010) LPSC basin terrains. A likely involvement of the LLD in the 41 st , Abstract # 2374. [12] Warner N. et al. (2011) JGR 116 , chaos basins formation is testified by the presence of E06003, doi:10.1029/2010JE003787. [13] Guallini L. et al. fractures intersecting their surface continuity, their par- (2011) LPSC 42 nd , Abstract #1433. [14] Guallini L. (2012, in tial erosion into mounds (that are locally underneath prep.) Ph.D. Thesis , Università d’Annunzio.

Fig. 1 a) Iani Chaos geologic and geomorphologic map. The white insets represent the three main depositional regions of the LLD (northern, central and southern); b) Ares Vallis outflow channels at the mouth of Iani Chaos; c) Perspective view of Iani units. Red lines represent fracturing systems crosscutting the LLD; d-o) Focusing on mapped units. See text for description and details.